Modern well logging techniques have led to the utilization of downhole pulsed neutron well logging systems. In particular, the measurement of earth formation thermal neutron decay times or thermal neutron lifetimes has become an important factor in determining residual oil saturations in earth formations in the vicinity of a well borehole. In copending application Ser. No. 182,172, filed Aug. 28, 1980 by Harold E. Peelman, and assigned to the assignee of the present invention, a thermal neutron decay time system is described which provides improved measurements of the thermal neutron lifetime of earth formations in the vicinity of a borehole. In the copending Peelman application, the thermal neutron lifetime measurements utilize a pulsed neutron source of the deuterium-tritium accelerator type and dual spaced detectors for making determinations of the thermal neutron lifetime of borehole and formation components of the thermal neutron lifetime simultaneously.
In making the measurements according to the techniques of the previously mentioned copending application, the pulsed neutron source is turned on and off at a rate of approximately 1000 pulses per second. Relatively short duration (10-30 microsecond) neutron pulses are used in this system. It has been found highly desirable to have precise control over the rise and fall time of the neutron pulses for making measurements according to the system of the aforementioned copending application. The present invention incorporates circuitry and techniques for assuring a very rapid rise time and very rapid fall time of neutron bursts emitted from a neutron generator of the deuterium-tritium accelerator type in a well borehole. The precise short rise and fall times of the neutron pulses are advantageous for thermal neutron decay time measurements and as well being advantageous for other types of pulsed neutron logging measurements such as carbon oxygen ratio inelastic scattering measurements.